Cation deficient transition metal sulfides have attracted increased attention due to their unique properties that arise from degenerate p-doping, particularly their localized surface plasmon resonance (LSPR) and related optical properties. Here, we present the first study of their electrocatalytic activity. We developed a facile one-pot method to prepare p-doped copper sulfide nanoplates with tunable LSPR at moderate temperature (below 100 °C) without any hot injection or rapid mixing step. The doping level was controlled by varying the concentration of cation precursor (Cu 2+ ) to finely tune the LSPR wavelength without changing the nanoplate size or morphology. Cu 2−x S nanoplates with three different doping levels were tested for their electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline solution. Importantly, increasing the concentration of free holes in Cu 2−x S significantly enhanced the ORR catalytic activity. Furthermore, to improve the electrical conductivity, the most heavily doped Cu 2−x S nanoplates were deposited on carbon black (Vulcan XC-72) and reduced graphene oxide (rGO), thereby leading to substantial enhancement of ORR steadystate current in both electrochemical and mass-transfer controlled potential regions. A calculation of average electron transfer number along with the measured peroxide yield indicated that both carbon black and rGO supported Cu 2−x S catalysts can provide a four-electron reduction pathway. The ORR catalytic activity of the Cu 2−x S nanoplates does not yet match that of stateof-the-art Pt/C catalysts. However, this work opens up new opportunities to apply p-doped copper chalcogenides as electrocatalysts for the ORR beyond conventional nonprecious metal catalysts based upon Fe, Co, N, and C.

Heterogeneous copper sulfide based nanostructures have attracted intense attention based on their potential to combine the plasmonic properties of copper-deficient copper sulfides with properties of other semiconductors and metals. In general, copper sulfides are versatile platforms for production of other materials by cation incorporation and exchange processes. However, the outcomes of subsequent cation exchange (CE) or incorporation processes involving nanoheterostructure (NH) templates have not been explored. In this work, we incorporate indium and tin into CuS-ZnS NHs. We demonstrate that the outcomes of cation incorporation are strongly influenced by heterocation identity and valence and by the presence of a Cu-extracting agent. The selectivity of cation incorporation depends upon both the cation itself and the heterodomains in which CE reactions take place. The final nanocrystals (NCs) emerge in many forms including homogeneous NCs, heterodimers, core@shell NHs and NHs with three different domains. This selective cation incorporation not only facilitates the preparation of previously unavailable metal sulfide NHs but also provides insight into mechanisms of CE reactions.

Synthesis of copper sulfide-based nanomaterials by cation incorporation into copper deficient copper sulfide (Cu2–x S) is of interest as a powerful means to obtain nanostructures with otherwise inaccessible combinations of size, shape, composition, and crystal phase. Incorporation of a heterocation (M) may produce heterogeneous Cu2–x S-MS nanocrystals (NCs) or homogeneous Cu-M-S alloys. However, the factors determining whether heterogeneous NCs or homogeneous alloy NCs are produced have not been fully elucidated. In this report, we incorporate diverse cations into covellite CuS nanoplatelet (NPl) templates in the presence of dodecanethiol (DDT). These cations are categorized by their valencies. We demonstrate that trivalent and tetravalent cations can be incorporated into reduced CuS NPls to produce homogeneous ternary alloy NPls, while the divalent cations cannot coexist with Cu+ ions in the Cu2–x S phase. In turn, the incorporation of divalent cations leads to formation of heterogeneous NPls and finally produces copper-free metal sulfide NPls. The cation valence selectivity arises from conflicts between charge balance and coordination between Cu+ and divalent cations. This study not only provides better understanding of the relationship among the composition, morphology, and crystal structure of copper sulfide-based nanomaterials but also provides a pathway to controllable synthesis of complex nanostructures.

Colloidal synthesis of copper tin sulfide (CTS) nanocrystals has been of great interest due to their potential for use in solution-processed photovoltaics containing only earth-abundant, low-toxicity elements. Postsynthetic incorporation of Sn into copper sulfide is an effective means of producing CTS, and Sn content can be controlled to some extent by the amount of Sn precursor provided. However, the oxidation (valence) state of Sn before and after doping and the effectiveness of Sn­(II) vs Sn­(IV) precursors remain topics of significant debate. Here, we demonstrate a step-growth method for preparing monodisperse copper tin sulfide (CTS) nanoplatelets (NPls). We show that Sn2+ ions, but not Sn4+ ions, can convert covellite to Cu3Sn x S4 NPls (x ≤ 1) without addition of any reducing or copper-extracting agent. In this case Sn2+ reduces the disulfide bond in covellite and the crystal structure evolves from covellite to kuramite. When dodecanethiol (DDT) is added prior to Sn, only Sn4+ ions, and not Sn2+ ions, are incorporated, ultimately producing mohite Cu2SnS3. Here, DDT can reduce the disulfide bonds and extract Cu from the lattice. When djurleite Cu2–x S was used as the template, only Sn4+, and not Sn2+, could be incorporated without use of DDT, and the extent of Sn incorporation was very limited, perhaps only filling Cu vacancies and not displacing any Cu from the lattice. Together with previous studies of CTS preparation from Cu2–x S using Sn4+ and DDT, these results provide a flexible array of methods of achieving a desired final CTS composition, crystal structure, and morphology. The ability to produce two different crystal phases of CTS, with corresponding differences in band gap and other properties, from the same covellite template while retaining the template size and morphology could be particularly valuable.

Mixed matrix membranes (MMMs) comprising size-sieving fillers dispersed in polymers exhibit diffusivity selectivity and may surpass the upper bound for gas separation, but their performance is limited by defects at the polymer/ filler interface. Herein, a fundamentally different approach employing a highly sorptive filler that is inherently less sensitive to interfacial defects is reported. Palladium nanoparticles with extremely high H 2 sorption are dispersed in polybenzimidazole at loadings near the percolation threshold, which increases both H 2 permeability and H 2 /CO 2 selectivity. Performance of these MMMs surpasses the state-of-the-art upper bound for H 2 /CO 2 separation with polymer-based membranes. The success of these sorption-enhanced MMMs for H 2 /CO 2 separation may launch a new research paradigm that taps the enormous knowledge of affinities between gases and nanomaterials to design MMMs for a wide variety of gas separations.